1. Field of the Invention
This invention relates generally to satellite positioning control systems, and more particularly to a satellite-based angular rate determination system and method.
2. Description of the Related Art
In the field of satellite positioning controls, continual, precise rate sensing is very important for precision satellite attitude control.
Currently, devices such as gyroscopes have been used in attitude control. However, mechanical gyroscopes have a limited life dictated by the life of the wheel suspension assembly and its sensitivity to influences such as temperature and vibration. Likewise, optical gyroscopes are limited by the life and stability of the light source, the resistance of the fiber or mirrors to ambient radiation, and other complex problems related to light and quantum limits.
In addition to gyroscopes, various forms of star sensors have been used to detect and control satellite attitude. For example, a static star sensor compares the stars seen over a relatively large area of space to a map of the universe, and calculates the direction of the sensor line of sight from the comparison of the image to the map. However, there are several limitations on this technology. First, the sensor field of view must be very large or the sensitivity of the sensor very high to obtain adequate information. Second, the brightness and position of the stars in the field of view must be compared to a very large catalog of information in order to decide which region of the heavens is in view. An alternative system, known as a tracker star sensor, follows the motion of a single star relative to the sensor base by rotating the sensor line of sight. The angle between the line of sight and the sensor base determines the base attitude. Such a system is limited by the fact that the device must be able to select a particular star.
It would therefore be advantageous to provide for a satellite attitude control system that is reliable, inexpensive and robust. It should be capable of determining angular rates without requiring the complicated hardware associated with a gyroscope. Likewise, any use of stellar references should not be dependent upon an extensive map of the universe or the ability to identify and track a single star.